1. Field of the Invention
The present invention relates to a solid-state image pick-up device such as a CCD, and more particularly to a solid-state image pick-up device including a channel stopper having a trench structure for isolating adjacent elements.
2. Description of the Related Art
FIG. 10 is a sectional view showing the main part of a conventional solid-state image pick-up device. A vertical transfer path (VCCD) 1 in an n region is formed in the surface portion of the semiconductor substrate of the solid-state image pick-up device, and an isolation is carried out between the adjacent vertical transfer paths 1 through a channel stopper (CS) 2 constituted by a p-type impurity layer. Moreover, a transfer and read electrode 3 comprising a polycrystalline silicon is provided on the surface of the semiconductor substrate.
The p-type impurity of the channel stopper 2 is diffused to the adjacent vertical transfer path 1 so that the effective transfer path (channel) width of the vertical transfer path 1 is decreased, resulting in a reduction in the channel potential of the vertical transfer path 1. This influence becomes more remarkable in manufacture in which the width of the vertical transfer path 1 is more reduced, which is referred to as a narrow channel effect.
FIG. 11 is a graph showing the narrow channel effect. For example, when the vertical transfer path 1 has a channel width of 0.5 μm, a potential of 5.8V is applied in a vertical transfer path in which a channel stopper (CS) is not provided. If the channel stopper is provided on either side, a potential of 5.3V is applied. If the channel stopper is provided on both sides, a potential of 4.6V is applied. Consequently, a charge is insufficiently read from a photodiode (pixel) to the vertical transfer path 1 or is insufficiently transferred along the vertical transfer path 1.
FIG. 12 is a chart in which a potential gradient around the channel stopper 2 shown in FIG. 10 is simulated, and the channel stopper 2 is formed around a position of 0.75 μm on an X coordinate. FIG. 12 is a chart showing a two-dimensional potential gradient obtained when a voltage of 0V is applied to the electrode 3 in FIG. 10, and FIG. 13 is a chart showing a two-dimensional potential gradient obtained when a voltage of −8V is applied to the electrode 3. Moreover, FIGS. 14 and 15 are charts showing a two-dimensional potential gradient and a one-dimensional potential which are obtained when a read voltage of +16V is applied to the electrode 3, and FIG. 16 is a chart showing a one-dimensional potential obtained when a charge is stored in the vertical transfer path.
From these drawings, it is apparent that the effect of an isolation obtained by the channel stopper 2 is small in the solid-state image pick-up device shown in FIG. 10. As shown in FIGS. 14 and 15, particularly, the potential of the channel stopper 2 also becomes deep by the influence of the potential of the transfer path, and a potential difference between the transfer path 1 and the channel stopper 2 is reduced and an isolation effect is small.
FIG. 17 is a view showing the structure of a conventional solid-state image pick-up device (for example, JP-A-2002-57381) in which an oxide film 4 is buried in the channel stopper 2 as compared with the solid-state image pick-up device shown in FIG. 10. FIGS. 18, 19 and 20 are views in which two-dimensional potential gradients obtained by application of voltages of 0V, −8V (transfer voltage) and +16V (read voltage) to the electrode 3 of the solid-state image pick-up device are simulated. Moreover, FIG. 21 is a chart showing a one-dimensional potential obtained when +16V is applied to the electrode 3 and FIG. 22 is a chart showing a one-dimensional potential obtained when a charge is stored in the vertical transfer path. The channel stopper 2 is formed to have a depth of 0.4 μm around a position of 0.75 μm on the X coordinate.
In the solid-state image pick-up device shown in FIG. 17, a channel potential is 11.5V in the application of 0V and is 4.1V in the application of −8V as shown in FIGS. 18 and 19 so that a degree of modulation (potential difference/voltage difference)=0.93 is obtained. On the other hand, in the solid-state image pick-up device shown in FIG. 10, a channel potential is 10.1V in the application of 0V and is 4.2V in the application of −8V as shown in FIGS. 12 and 13 so that a degree of modulation=0.74 is obtained. More specifically, it can be guessed that an oxide film 4 is buried in the channel stopper 2 so that the p-type impurity layer is eliminated, resulting in the prevention of a reduction in a channel potential and a relaxation in the influence of the narrow channel effect.
In the conventional solid-state image pick-up device shown in FIG. 17, however, in the case in which a charge is stored in the vertical transfer path 1, there is a problem in that a potential difference between an interface 5 of the channel stopper 2 and the signal charge of the vertical transfer path 1 is reduced (see FIGS. 16 and 22) and a charge (an electron in the solid-state image pick-up device shown in the drawing) is apt to be trapped into the interface 5. When the charge is trapped into the interface 5, the trapped charge gets into the vertical transfer path 1 of a next packet at a constant probability, thereby deteriorating a transfer efficiency.
In a recent solid-state image pick-up device mounting several million pixels, a transfer is carried out in one thousand stages or more through a one-time transfer along a vertical transfer path. For this reason, if one electron gets into an empty packet every transfer in one stage, for example, at least one thousand electrons get into the empty packet when the same packet is read from the vertical transfer path. Consequently, the picture quality of a pick-up image is considerably deteriorated.
Furthermore, the interface 5 is depleted. Therefore, there is also a problem in that the generation of a dark current from a dangling bond and the generation of a white scratch from a high-speed GR center (a heavy metal atom) are increased remarkably.